To meet the dramatic growth in Internet demand, the optical industry must find new methods to increase the total capacity of existing fibre networks and ensure these new technologies are economically efficient, operationally simple, and scalable.
As optical data transmission speeds increase and optical transmission is promoted for short distances such as in data centres, it becomes more important to push more data through fibres. For very high transmission speeds it becomes unrealistic to transmit over a single carrier. For example, for 1 Tb/s transmission, even with 100 GS/s Analogue to Digital Conversion (ADC) technology, very sophisticated quadrature amplitude modulation (PM-1024QAM) would be required. This leads to poor sensitivity, phase noise issues, an impact on non-linearity, hardware problems, and a very short reach.
The solution is for information to be distributed over a few subcarriers spaced as closely as possible, together forming a “superchannel”. Each subcarrier can work at a lower rate which is compatible with current ADCs and Digital Signal Processors (DSPs).
A concept widely discussed for achieving very high transmission speeds is flexible wavelength transmission (sometimes known as flexgrid). For 400 G transmission, dual polarisation quadrature phase shift keying with 4 subcarriers and DP-16 quadrature amplitude modulation (QAM) with two subcarriers are options under consideration. These options become spectrally efficient by matching the channel size with the bandwidth of the signal being transmitted.
However, where a number of subcarriers are used, their signals need to be combined before transmission into the optical fibre, so that all of the subcarriers are multiplexed.
Similarly, in a receiver the subcarriers need to be demultiplexed. This can be carried out using passive splitters/combiners, which could be incorporated into the modulator chip, but such components are relatively lossy. An alternative is to use multiplexers/demultiplexers external to the chip, but this increases the footprint of the transmitter or receiver.